1. Technical Field
The present invention relates to an electronic endoscopic apparatus and a control method thereof.
2. Related Art
For CCD type or CMOS type image sensors (imaging devices) to be mounted in a digital camera, the number of pixels has been increased such that a subject image can be imaged with high definition. An image sensor mounted in a front-end part of a scope of an electronic endoscopic apparatus is no exception to the trend of the increasing the number of pixels, and the number of pixels has been increased for the purpose of improving the descriptiveness of a minute structure such as, for example, blood vessels.
Specifically, in an area that a doctor is interested in, for example, a subject or an interior wall of a biological body located at the center of a screen and brightened by an illuminating light (hereinafter, referred to as an “interest area”), the increase of the number of pixels benefits as the high definition of a captured image. However, because the pixel size of each pixel is decreased due to the increase of the number of pixels, there is a problem in that the amount of light capable of being received in a light receiving surface per one pixel is decreased, and thus, the sensitivity of a captured image is sacrificed.
Meanwhile, in a non-interest area, for example, the inside in a biological body or a periphery of a screen, in which the amount of the illuminating light is not sufficient, it is more important to realize a change in color such as, for example, a rube, or a change in structure such as, for example, a polyp than to inspect a high definition image in detail. That is, a captured image become required that places priority to a screening to find a new interest area. Therefore, the sensitivity of a captured image becomes more important than the precision of the captured image.
In case of a digital camera, as disclosed in, for example, JP-A-2008-072501 and JP-2007-251694, a high definition image is generated based on detection signals of each individual pixel in the bright area within one imaging screen, and pixel addition is performed to add detection signals of a plurality of pixels while sacrificing the precision in order to place priority to the sensitivity in the dark area within the screen.
JP-A-H11-313247 discloses that the pixel addition performed in the digital camera is also performed in an electronic endoscope. However, in the electronic endoscope, even if whether to perform the pixel addition only based on the brightness or the number of pixel addition is determined, it is not the pixel addition that meets an intended use of the electronic endoscope. That is because the photographing environment for using the electronic endoscope is substantially different from that for the digital camera.
The front-end part of the scope of an image sensor built-in electronic endoscope is inserted and used within a biological body. That is, the front-end part of the scope is used in a dark environment. For this reason, the imaging is performed while illuminating light is illuminated to an area to be observed from a light source built in the front-end part of the scope or a light guide inserted through the endoscopic scope.
The brightness of the area illuminated by the illuminating light is greatly changed depending on whether a direction where the front-end part of the scope is headed, i.e., the illuminating direction of the illuminating light faces a wall within the biological body or an inner side within the biological body. The ratio of the brightness may be, for example, a ratio of 2000:1. With the endoscope, an observation at the time of precise inspecting that carefully observes a region for a long time and an observation at the time of screening time that observes the images obtained while inserting the endoscopic scope into the biological body, are often alternately performed.
In the observation at the time of screening when the brightness is occasionally changed from time to time in the above described ratio of 2000:1, it is difficult to obtain a good observation image. Therefore, the light emitting amount is controlled in the electronic endoscope in order to reduce the change of the brightness according to the insertion of the endoscopic scope. Further, in the observation at the time of precise inspecting, an enlarged image is observed by locating the front-end part of the scope close to the interest area such as, for example, a lesion part to illuminate the illuminating light.
Because such a photographing environment distinct to the endoscope exist, it is required to perform the pixel addition control by adding a factor in addition to the brightness suitable for an intended use of the endoscope, rather than to determine the pixel addition number and a pixel addition area merely based on the brightness.